The Devil's Dictionaryby Ambrose Bierce

X in our alphabet being a needless letter has an added invincibility to the attacks of the spelling reformers, and like them, will doubtless last as long as the language

X is the sacred symbol of ten dollars, and in such words as Xmas, Xn, etc., stands for Christ, not, as is popular supposed, because it represents a cross, but because the corresponding letter in the Greek alphabet is the initial of his name — Xristos. If it represented a cross it would stand for St. Andrew, who "testified" upon one of that shape. In the algebra of psychology x stands for Woman's mind. Words beginning with X are Grecian and will not be defined in this standard English dictionary.